Co-pending U.S. patent application Ser. No. 08/500,441, filed Jul. 10, 1995, now U.S. Pat. No. 5,598,413 issued Jan. 28, 1997, entitled: "FOUR WIRE, HALF-RATE ARCHITECTURE WITH EMBEDDED DIFFERENTIAL DELAY COMPENSATION FOR EXTENDING RANGE OF BASIC RATE ISDN COMMUNICATIONS," by M. Sansom et al, assigned to the assignee of the present application and the disclosure of which is herein incorporated (hereinafter referred to as the '441 application), describes a one-half rate ISDN demultiplexing--multiplexing architecture for solving a communication range extension problem resulting from an American National Standards Institute (ANSI) standard T1.601 governing ISDN communications.
In particular, ANSI standard T1.601 for 2B1Q modulation, two-wire, full-duplex data transfer with echo cancellation requires that currently installed ISDN basic rate digital subscriber lines (having a data rate of 144 kbps, with bidirectional data payload, plus overhead maintenance channels) must not exceed a two-wire loop loss of 42 dB at 40 kHz, or 1300 ohms, resistive. Such a requirement effectively limits the operational range of such a two-wire loop to approximately 15.2 kft, using No. 26 (American Wire Gauge) wire, and commercially available ISDN transceiver equipment. Extending ISDN communications to customers geographically located beyond this range requires that the service provider either install repeaters in the loop, or use a different communication medium, such as a T1 carrier fiber optic link, with both solutions being unattractive from a standpoint of cost, installation and maintenance.
In accordance with the invention described in the '441 application, this problem is solved by an ISDN communications architecture employing a pair of demultiplexed half-rate, integrated services digital network (ISDN) channels, having an out-of-band maintenance channel for conveying differential delay compensation information, to extend the normal range of ISDN basic rate digital subscriber lines to distances (on the order of 25 kft) well beyond those currently possible (typically 15.2 kft) using a repeaterless two-wire transmission path.
A problem similar to that described above for ISDN basic rate digital subscriber lines is encountered in (four-wire) DDS communications. In particular, if a telecommunications loop is to be a candidate for DDS assignment, it must comply with AT&T Technical Reference Publication No. 62310, which requires that the measured insertion loss in one direction between an Office Channel Unit Data Port (OCU DP) and the customer premises point of demarcation be less than 34 dB, when measured at the Nyquist frequency (1/2 the data rate for alternate mark inversion (AMI) circuits) and terminated by a 135 ohm impedance. This equates to approximately 21 kft of 24 AWG (American Wire Gauge) copper wire, or 18 kft of mixed gauge 26 and 24 AWG copper wire.
Although many OCU DP units can accommodate 45 dB of signal loss over copper wire loops, the requirement for the customer-owned equipment in terms of receiver sensitivity is only 38 dB, so that extending DDS communications to customers premises located beyond the 34 dB range requires that the service provider install either a loop repeater or a T1 Digital Loop Carrier system, each of which entails a substantial cost and installation penalty, as described above.
DDS loop repeaters are currently deployed on circuits operating at data rates of 56 kbps, 56 kbps with secondary channel, and 64 kbps. The effective communication range of circuits operating at data rates below 56 kbps can be extended to greater distances and therefore seldom require the use of a DDS repeater. A DDS repeater is powered from the loop and requires specially equipped OCU DP units or additional line powering modules, as well as a repeater housing or environmentally hardened case that can be mounted on a pole or within a subterranean enclosure.
In addition to the expense and time associated with the installation of the repeater and associated components is the expense incurred in the course of periodic maintenance of the repeater enclosure. While Digital Loop Carrier (DLC) systems offer a favorable solution to the dilemmas posed in DDS range extension when available, their cost cannot be justified on an individual circuit basis, due to the considerable investment for the service provider in terms of cost and installation time.